This invention relates to an apparatus and method for bending tubular stock into the shape of a U to form heat transfer tubes suitable for use in a heat exchanger. More particularly but not exclusively, it relates to an apparatus and method capable of forming U-shaped heat transfer tubes for use in a heat exchanger of a pressurized water reactor. It also relates to a heat exchanger employing such tubes.
A steam generator in a heat exchanger for a pressurized water reactor comprises an array of heat transfer tubes formed from a plurality of U-shaped tubes (referred to below as U-bend tubes) of differing bending radius.
FIGS. 1A-1C schematically illustrate an array of heat transfer tubes, and FIGS. 2A and 2B are plan views of support plates for heat transfer tubes.
As shown in FIG. 1A, the upper portions of the heat transfer tubes generally form a hemisphere. On the innermost portion of the hemisphere, a plurality of U-bend tubes 1.sub.1, 1.sub.1, . . . having the smallest bending radius are spaced at equal intervals along a Z axis, which is perpendicular to the bending planes of the tubes 1.sub.1, 1.sub.1, . . . . On the outside of tubes 1.sub.1, 1.sub.1, . . . are arranged a plurality of U-bend tubes 1.sub.2, 1.sub.2, . . . , U-bend tubes 1.sub.3, 1.sub.3, . . . , etc. of successively larger bending radius. These tubes having the same bending radius, like tubes 1.sub.1, are spaced at equal intervals in the Z direction. The spacing between the larger radius tubes 1.sub.2, 1.sub.3, etc. in the Z direction is the same as between the smallest radius tubes 1.sub.1.
FIGS. 2A and 2B illustrate two conventional arrangements of U-bend tubes in a steam generator. The arrangement of FIG. 2A is referred to as a rectangular array, while the arrangement of FIG. 2B is referred to as a triangular array. In the rectangular array, tubes of successively larger bending radius are disposed in the same bending plane. For example, in FIG. 2A, tubes 1.sub.1 -1.sub.5 all lie in a common bending plane. In the triangular array, tubes of successively larger bending radius are disposed in bending planes which are staggered from one another. Thus, in FIG. 2B, tubes 1.sub.1, 1.sub.3, 1.sub.5, and 1.sub.7 lie in a first bending plane, while tubes 1.sub.2, 1.sub.4, and 1.sub.6 lie in a second bending plane spaced midway between two of the first bending planes.
In either arrangement, the number of tubes progressively increases from the ends of the array in the Z direction towards the center. Below the hemispherical portion, the tubes extend straight downwards.
Namely, a first series of U-bend tubes of a first nominal bending radius is arranged in a row with the U-bends aligned in the Z direction. Then, a second series of U-bend tubes smaller in number than the first series of tubes and each having a second nominal bending radius which is larger than the first nominal bending radius is arranged in a row with the U-bends of the second series of tubes aligned in the Z direction. Each of the U-bends of the second series of tubes is concentric with respect to one of the U-bends of the first series of tubes. Subsequent series of U-bend tubes are arranged in a similar manner, with the number of tubes in each series decreasing and the nominal bending diameter increasing as the distance from the first series of tubes increases. In this manner, a hemispherical portion is formed at the top portion of an assembly of the U-bend tubes.
A steam generator of this type commonly employs more than 100 different types of tubes 1.sub.1, 1.sub.2, . . . , etc. of differing bending radius. Therefore, at the center of the array in Z direction, more than 100 different U-bend tubes are concentrically arranged in the same bending plane. See FIG. 1C. The total number of tubes in a steam generator of this type may be more than 7000.
In a steam generator of a heat exchanger for a pressurized water reactor, it is extremely important to secure the heat transfer tubes to prevent them from being damaged. For this reason, as shown in FIG. 1A, a plurality of levels of support plates 4 are used to secure the straight portions of the tubes except the hemispherical portion. However, it is impossible for the support plates 4 to secure the tubes in the hemispherical portion, so V-shaped antivibration bars 2 are inserted into the gaps between adjacent bending planes to secure the bending portions of the tubes, except for the tubes having smaller bending radius, since these tubes do not project far above the support plate 4 and so are relatively stiff.
For example, at the center of the hemispherical portion in the Z direction, a plurality of antivibration bars 2.sub.1, 2.sub.2, etc. are disposed at different levels. The antivibration bars 2 are typically metal bars having a rectangular cross section. The outer ends of the antivibration bars 2 are secured by holders 3.sub.1, 3.sub.2, etc. which extend in curves along the surface of the hemispherical portion.
The U-bends of heat transfer tubes of this type must have a high dimensional accuracy. Therefore, they are frequently manufactured by a bending process employing a die. Two tube bending methods using a die are rotary draw bending, illustrated in FIG. 3A, and compression bending, illustrated in FIG. 3B.
In rotary draw bending, as shown in FIG. 3A, a bending die and a clamp 6 for securing a workpiece W on the bending die 5 are employed. A groove corresponding to the external shape of the workpiece W is formed in the peripheral surface of the bending die 5. A groove corresponding to the external shape of the workpiece W is also formed in the clamp 6.
The workpiece W is grasped between the bending die 5 and the clamp 6, and in this state, the bending die 5 and the clamp 6 are synchronously rotated about the center of the bending die 5. As a result, the workpiece W is pressed into the groove of the bending die 5 and is suitably bent. At this time, the clamp 6 draws the workpiece W to move it in its axial direction, and thus it is called "rotary draw bending".
In compression bending, as shown in FIG. 3B, a roller 7 is used instead of a clamp 6. A groove corresponding to the external shape of the workpiece W is formed in the roller 7 around its entire circumference. With the workpiece W held between the bending die 5 and the roller 7, the roller 7 is rolled around the periphery of the bending die 5, and the workpiece W is pressed into the groove of the bending die 5.
Many methods of tube bending have been proposed (see Japanese Published Unexamined Patent Application Nos. 50-29465, 58-159923, and 58-159924 and Japanese Published Unexamined Utility Model Application No. 58-185324, for example). Of these methods, those employing a die can all be classified as either rotary draw bending or compression bending.